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97-697: The functions of tears include lubricating the eyes (basal tears), removing irritants (reflex tears), and also aiding the immune system . Tears also occur as a part of the body's natural pain response. Emotional secretion of tears may serve a biological function by excreting stress-inducing hormones built up through times of emotional distress. Tears have symbolic significance among humans . Tears are made up of three layers: lipid, aqueous, and mucous. Tears are composed of water , salts , antibodies , and lysozymes (antibacterial enzymes); though composition varies among different tear types. The composition of tears caused by an emotional reaction differs from that of tears as

194-564: A lysosome to form a phagolysosome . The pathogen is killed by the activity of digestive enzymes or following a respiratory burst that releases free radicals into the phagolysosome. Phagocytosis evolved as a means of acquiring nutrients , but this role was extended in phagocytes to include engulfment of pathogens as a defense mechanism. Phagocytosis probably represents the oldest form of host defense, as phagocytes have been identified in both vertebrate and invertebrate animals. Neutrophils and macrophages are phagocytes that travel throughout

291-465: A "self" receptor called a major histocompatibility complex (MHC) molecule. There are two major subtypes of T cells: the killer T cell and the helper T cell . In addition there are regulatory T cells which have a role in modulating immune response. Killer T cells are a sub-group of T cells that kill cells that are infected with viruses (and other pathogens), or are otherwise damaged or dysfunctional. As with B cells, each type of T cell recognizes

388-423: A chemical barrier following menarche , when they become slightly acidic , while semen contains defensins and zinc to kill pathogens. In the stomach , gastric acid serves as a chemical defense against ingested pathogens. Within the genitourinary and gastrointestinal tracts, commensal flora serve as biological barriers by competing with pathogenic bacteria for food and space and, in some cases, changing

485-422: A condition known as "missing self". This term describes cells with low levels of a cell-surface marker called MHC I ( major histocompatibility complex )—a situation that can arise in viral infections of host cells. Normal body cells are not recognized and attacked by NK cells because they express intact self MHC antigens. Those MHC antigens are recognized by killer cell immunoglobulin receptors, which essentially put

582-584: A different antigen. Killer T cells are activated when their T-cell receptor binds to this specific antigen in a complex with the MHC Class I receptor of another cell. Recognition of this MHC:antigen complex is aided by a co-receptor on the T cell, called CD8 . The T cell then travels throughout the body in search of cells where the MHC I receptors bear this antigen. When an activated T cell contacts such cells, it releases cytotoxins , such as perforin , which form pores in

679-400: A different explanation for the tears, saying "There are not many brute beasts that can weep, but such is the nature of the crocodile that, to get a man within his danger, he will sob, sigh, and weep as though he were in extremity, but suddenly he destroyeth him." In this version, the crocodile pretends to be in distress to lure prey into a false sense of security. However, Topsell also refers to

776-533: A diminished effect and may result in lower antibody production, and a lower immune response, than would be noted in a well-rested individual. Additionally, proteins such as NFIL3 , which have been shown to be closely intertwined with both T-cell differentiation and circadian rhythms , can be affected through the disturbance of natural light and dark cycles through instances of sleep deprivation. These disruptions can lead to an increase in chronic conditions such as heart disease, chronic pain, and asthma. In addition to

873-454: A full translucent third eyelid called a nictitating membrane , while others have a vestigial nictitating membrane. The membrane works to protect and moisten the eyelid while maintaining visibility. It also contributes to the aqueous portion of the tear film and possibly immunoglobulins. Humans and some primates have a much smaller nictitating membrane; this may be because they do not capture prey or root vegetation with their teeth, so that there

970-491: A hyperactive immune system attacking normal tissues as if they were foreign organisms. Common autoimmune diseases include Hashimoto's thyroiditis , rheumatoid arthritis , diabetes mellitus type 1 , and systemic lupus erythematosus . Immunology covers the study of all aspects of the immune system. The immune system protects its host from infection with layered defenses of increasing specificity. Physical barriers prevent pathogens such as bacteria and viruses from entering

1067-492: A lack of overflow tears ( Alacrima ) during emotional crying. Obstruction of the punctum , nasolacrimal canal , or nasolacrimal duct can cause even normal levels of the basal tear to overflow onto the face ( Epiphora ), giving the appearance of constant psychic tearing. This can have significant social consequences. Pseudobulbar affect (PBA) is a condition involving episodic uncontrollable laughter or crying. PBA mostly occurs in people with neurological injuries affecting how

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1164-399: A link between the bodily tissues and the innate and adaptive immune systems, as they present antigens to T cells , one of the key cell types of the adaptive immune system. Granulocytes are leukocytes that have granules in their cytoplasm. In this category are neutrophils, mast cells, basophils, and eosinophils. Mast cells reside in connective tissues and mucous membranes and regulate

1261-515: A meaningless description ending with the words "And the tears of it are wet". Shakespeare's contemporary Edmund Spenser also refers to the story in The Faerie Queene , writing of the "cruel crafty" creature "which, in false grief, hiding his harmful guile / Doth weep full sore, and sheddeth tender tears. In Henry Purcell 's 1688 opera Dido and Aeneas , (librettist Nahum Tate ), when Aeneas tells Dido he must abandon her to found Rome on

1358-460: A pro-inflammatory state through the production of the pro-inflammatory cytokines interleukin-1, interleukin-12 , TNF-alpha and IFN-gamma . These cytokines then stimulate immune functions such as immune cell activation, proliferation, and differentiation . During this time of a slowly evolving adaptive immune response, there is a peak in undifferentiated or less differentiated cells, like naïve and central memory T cells. In addition to these effects,

1455-412: A reaction to irritants, such as onion fumes, dust, or allergens. Emotional tears contain higher concentrations of stress hormones such as adrenocorticotropic hormone and leucine enkephalin (a natural pain killer), which suggests that emotional tears play a biological role in balancing stress hormone levels. The lacrimal glands secrete lacrimal fluid, which flows through the main excretory ducts into

1552-491: A reduced ability to destroy pathogens, is an example of an inherited, or congenital, immunodeficiency . AIDS and some types of cancer cause acquired immunodeficiency. Overactive immune responses form the other end of immune dysfunction, particularly the autoimmune diseases . Here, the immune system fails to properly distinguish between self and non-self, and attacks part of the body. Under normal circumstances, many T cells and antibodies react with "self" peptides. One of

1649-512: A researcher at the University of Florida, decided to test the story that crocodiles or their close relatives alligators and caimans were likely to "weep" while feeding. Studying animals in Florida's St. Augustine Alligator Farm Zoological Park , Vliet recorded seven caimans feeding. He chose to use caimans rather than crocodiles because at the sanctuary they could be observed feeding on dry land. Five of

1746-471: A single MHC:antigen molecule. Helper T cell activation also requires longer duration of engagement with an antigen-presenting cell. The activation of a resting helper T cell causes it to release cytokines that influence the activity of many cell types. Cytokine signals produced by helper T cells enhance the microbicidal function of macrophages and the activity of killer T cells. In addition, helper T cell activation causes an upregulation of molecules expressed on

1843-460: A specific foreign antigen. This antigen/antibody complex is taken up by the B cell and processed by proteolysis into peptides . The B cell then displays these antigenic peptides on its surface MHC class II molecules. This combination of MHC and antigen attracts a matching helper T cell, which releases lymphokines and activates the B cell. As the activated B cell then begins to divide , its offspring ( plasma cells ) secrete millions of copies of

1940-529: A tailored response to each stimulus by learning to recognize molecules it has previously encountered. Both use molecules and cells to perform their functions. Nearly all organisms have some kind of immune system. Bacteria have a rudimentary immune system in the form of enzymes that protect against viral infections. Other basic immune mechanisms evolved in ancient plants and animals and remain in their modern descendants. These mechanisms include phagocytosis , antimicrobial peptides called defensins , and

2037-414: A wide variety of pathogens , from viruses to bacteria , as well as cancer cells , parasitic worms , and also objects such as wood splinters , distinguishing them from the organism's own healthy tissue . Many species have two major subsystems of the immune system. The innate immune system provides a preconfigured response to broad groups of situations and stimuli. The adaptive immune system provides

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2134-482: Is a colloquialism for Bogorad's syndrome , an uncommon consequence of recovery from Bell's palsy in which faulty regeneration of the facial nerve causes people to shed tears while eating. Bogorad's syndrome, also known as "Crocodile Tears Syndrome", is an uncommon consequence of nerve regeneration subsequent to Bell's palsy or other damage to the facial nerve . Efferent fibers from the superior salivary nucleus become improperly connected to nerve axons projecting to

2231-427: Is a manner of a long serpent, as I have said before. And in the night they dwell in the water, and on the day upon the land, in rocks and in caves. And they eat no meat in all the winter, but they lie as in a dream, as do the serpents. These serpents slay men, and they eat them weeping; and when they eat they move the over jaw, and not the nether jaw, and they have no tongue. A later writer, Edward Topsell , provided

2328-409: Is a transient immunodepression, where the number of circulating lymphocytes decreases and antibody production declines. This may give rise to a window of opportunity for infection and reactivation of latent virus infections, but the evidence is inconclusive. During exercise there is an increase in circulating white blood cells of all types. This is caused by the frictional force of blood flowing on

2425-446: Is activated by complement binding to antibodies that have attached to these microbes or the binding of complement proteins to carbohydrates on the surfaces of microbes . This recognition signal triggers a rapid killing response. The speed of the response is a result of signal amplification that occurs after sequential proteolytic activation of complement molecules, which are also proteases. After complement proteins initially bind to

2522-527: Is affected by sleep and rest, and sleep deprivation is detrimental to immune function. Complex feedback loops involving cytokines , such as interleukin-1 and tumor necrosis factor-α produced in response to infection, appear to also play a role in the regulation of non-rapid eye movement ( REM ) sleep. Thus the immune response to infection may result in changes to the sleep cycle, including an increase in slow-wave sleep relative to REM sleep. In people with sleep deprivation, active immunizations may have

2619-502: Is also recognized by the helper cell's CD4 co-receptor, which recruits molecules inside the T cell (such as Lck ) that are responsible for the T cell's activation. Helper T cells have a weaker association with the MHC:antigen complex than observed for killer T cells, meaning many receptors (around 200–300) on the helper T cell must be bound by an MHC:antigen to activate the helper cell, while killer T cells can be activated by engagement of

2716-549: Is an immune response that damages the body's own tissues. It is divided into four classes (Type I – IV) based on the mechanisms involved and the time course of the hypersensitive reaction. Type I hypersensitivity is an immediate or anaphylactic reaction, often associated with allergy. Symptoms can range from mild discomfort to death. Type I hypersensitivity is mediated by IgE , which triggers degranulation of mast cells and basophils when cross-linked by antigen. Type II hypersensitivity occurs when antibodies bind to antigens on

2813-415: Is an important feature of cellular innate immunity performed by cells called phagocytes that engulf pathogens or particles. Phagocytes generally patrol the body searching for pathogens, but can be called to specific locations by cytokines. Once a pathogen has been engulfed by a phagocyte, it becomes trapped in an intracellular vesicle called a phagosome , which subsequently fuses with another vesicle called

2910-616: Is cheating on him. If that the earth could teem with woman's tears, Each drop she falls would prove a crocodile. He also refers to the version about tricking prey in Henry VI, Part 2 , Act III, Scene i, in which a character refers to the faked emotions of the Duke of Gloucester: "Gloucester's show / Beguiles him, as the mournful crocodile / With sorrow, snares relenting passengers." In Antony and Cleopatra , Act II, Scene vii, Mark Antony chides Lepidus, who has asked him what crocodiles are like, with

3007-517: Is evidence for an interpersonal function of crying as tears express a need for help and foster willingness to help in an observer. Some modern psychotherapy movements such as Re-evaluation Counseling encourage crying as beneficial to health and mental well-being. An insincere display of grief or dishonest remorse is sometimes called crocodile tears in reference to an Ancient Greek anecdote that crocodiles would pretend to weep while luring or devouring their prey. In addition, "crocodile tears syndrome"

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3104-530: Is mediated by transmembrane proteins known as toll-like receptors (TLRs). TLRs share a typical structural motif, the leucine rich repeats (LRRs) , which give them a curved shape. Toll-like receptors were first discovered in Drosophila and trigger the synthesis and secretion of cytokines and activation of other host defense programs that are necessary for both innate or adaptive immune responses. Ten toll-like receptors have been described in humans. Cells in

3201-405: Is no evolutionary advantage of the third eyelid. The trigeminal V 1 (fifth cranial) nerve bears the sensory pathway of the tear reflexes. When the trigeminal nerve is cut, tears from reflexes will stop, while emotional tears will not. The great (superficial) petrosal nerve from cranial nerve VII provides autonomic innervation to the lacrimal gland. It is responsible for the production of much of

3298-424: Is often associated with babies and children. Some cultures consider crying to be undignified and infantile, casting aspersions on those who cry publicly, except if it is due to the death of a close friend or relative. In most Western cultures, it is more socially acceptable for women and children to cry than men, reflecting masculine sex-role stereotypes. In some Latin regions, crying among men is more acceptable. There

3395-497: Is one of the first responses of the immune system to infection, but it can appear without known cause. Crocodile tears Crocodile tears , or superficial sympathy , is a false, insincere display of emotion such as a hypocrite crying fake tears of grief . The phrase derives from an ancient belief that crocodiles shed tears while consuming their prey, and as such is present in many modern languages, especially in Europe where it

3492-452: Is the lacrimal punctum. From the punctum, tears will enter the lacrimal sac , then on to the nasolacrimal duct , and finally into the nasal cavity . An excess of tears, as caused by strong emotion , can cause the nose to run. Quality of vision is affected by the stability of the tear film. There are three basic types of tears: basal, reflex and emotional. Some mammals, such as cats , camels , polar bears , seals and aardvarks , have

3589-504: Is to generate active forms of the inflammatory cytokines IL-1β and IL-18. The complement system is a biochemical cascade that attacks the surfaces of foreign cells. It contains over 20 different proteins and is named for its ability to "complement" the killing of pathogens by antibodies . Complement is the major humoral component of the innate immune response. Many species have complement systems, including non- mammals like plants, fish, and some invertebrates . In humans, this response

3686-559: The Bibliotheca by early medieval theologian Photios . Photios uses the story to illustrate the Christian concept of repentance. The story is repeated in bestiaries such as De bestiis et aliis rebus . This tale was first spread widely in English in the stories of the travels of Sir John Mandeville in the 14th century. In that country and by all Inde be great plenty of cockodrills, that

3783-402: The "professional" phagocytes ( macrophages , neutrophils , and dendritic cells ). These cells identify and eliminate pathogens, either by attacking larger pathogens through contact or by engulfing and then killing microorganisms. The other cells involved in the innate response include innate lymphoid cells , mast cells , eosinophils , basophils , and natural killer cells . Phagocytosis

3880-414: The complement system . Jawed vertebrates , including humans, have even more sophisticated defense mechanisms, including the ability to adapt to recognize pathogens more efficiently. Adaptive (or acquired) immunity creates an immunological memory leading to an enhanced response to subsequent encounters with that same pathogen. This process of acquired immunity is the basis of vaccination . Dysfunction of

3977-573: The endothelial cell surface and catecholamines affecting β-adrenergic receptors (βARs). The number of neutrophils in the blood increases and remains raised for up to six hours and immature forms are present. Although the increase in neutrophils (" neutrophilia ") is similar to that seen during bacterial infections, after exercise the cell population returns to normal by around 24 hours. The number of circulating lymphocytes (mainly natural killer cells ) decreases during intense exercise but returns to normal after 4 to 6 hours. Although up to 2% of

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4074-449: The exoskeleton of insects, the shells and membranes of externally deposited eggs, and skin are examples of mechanical barriers that are the first line of defense against infection. Organisms cannot be completely sealed from their environments, so systems act to protect body openings such as the lungs , intestines , and the genitourinary tract . In the lungs, coughing and sneezing mechanically eject pathogens and other irritants from

4171-458: The innate immune system , such as dendritic cells, macrophages, monocytes, neutrophils, and epithelial cells, to identify two classes of molecules: pathogen-associated molecular patterns (PAMPs), which are associated with microbial pathogens , and damage-associated molecular patterns (DAMPs), which are associated with components of host's cells that are released during cell damage or cell death. Recognition of extracellular or endosomal PAMPs

4268-453: The lacrimal glands , causing one to shed tears ( lacrimate ) on the side of the palsy during salivation while smelling foods or eating. It is presumed that this would cause salivation while crying due to the inverse improper connection of the lacrimal nucleus to the salivary glands, but this would be less noticeable. The condition was first described in 1926 by its namesake, Russian neuropathologist F. A. Bogorad, in an article titled "Syndrome of

4365-492: The lymphoid lineage . These cells are defined by the absence of antigen-specific B- or T-cell receptor (TCR) because of the lack of recombination activating gene . ILCs do not express myeloid or dendritic cell markers. Natural killer cells (NK cells) are lymphocytes and a component of the innate immune system that does not directly attack invading microbes. Rather, NK cells destroy compromised host cells, such as tumor cells or virus-infected cells, recognizing such cells by

4462-627: The nervous systems. The immune system also plays a crucial role in embryogenesis (development of the embryo), as well as in tissue repair and regeneration . Hormones can act as immunomodulators , altering the sensitivity of the immune system. For example, female sex hormones are known immunostimulators of both adaptive and innate immune responses. Some autoimmune diseases such as lupus erythematosus strike women preferentially, and their onset often coincides with puberty . By contrast, male sex hormones such as testosterone seem to be immunosuppressive . Other hormones appear to regulate

4559-514: The respiratory tract . The flushing action of tears and urine also mechanically expels pathogens, while mucus secreted by the respiratory and gastrointestinal tract serves to trap and entangle microorganisms . Chemical barriers also protect against infection. The skin and respiratory tract secrete antimicrobial peptides such as the β- defensins . Enzymes such as lysozyme and phospholipase A2 in saliva , tears, and breast milk are also antibacterials . Vaginal secretions serve as

4656-399: The B cell surface and recognizes native (unprocessed) antigen without any need for antigen processing . Such antigens may be large molecules found on the surfaces of pathogens, but can also be small haptens (such as penicillin) attached to carrier molecule. Each lineage of B cell expresses a different antibody, so the complete set of B cell antigen receptors represent all the antibodies that

4753-527: The Crocodile Tears" (alternatively, "The Symptom of the Crocodile Tears") that argued the tears were caused by the act of salivation. Keratoconjunctivitis sicca , known in the vernacular as dry eye, is a very common disorder of the tear film. Despite the eyes being dry, those affected can still experience watering of the eyes, which is, in fact, a response to irritation caused by the original tear film deficiency. Lack of Meibomian gland secretion can mean that

4850-474: The Italian Peninsula, she proclaims, "Thus on the fatal banks of Nile, / Weeps the deceitful crocodile." While crocodiles can and do generate tears, the tears are not linked to emotion. The fluid from their tear ducts functions to clean and lubricate the eye, and is most prominent and visible when crocodiles have been on dry land for a while. In the case of American crocodiles and saltwater crocodiles ,

4947-587: The T cell's surface, such as CD40 ligand (also called CD154 ), which provide extra stimulatory signals typically required to activate antibody-producing B cells. Gamma delta T cells (γδ T cells) possess an alternative T-cell receptor (TCR) as opposed to CD4+ and CD8+ (αβ) T cells and share the characteristics of helper T cells, cytotoxic T cells and NK cells. The conditions that produce responses from γδ T cells are not fully understood. Like other 'unconventional' T cell subsets bearing invariant TCRs, such as CD1d -restricted natural killer T cells , γδ T cells straddle

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5044-565: The antibody that recognizes this antigen. These antibodies circulate in blood plasma and lymph , bind to pathogens expressing the antigen and mark them for destruction by complement activation or for uptake and destruction by phagocytes . Antibodies can also neutralize challenges directly, by binding to bacterial toxins or by interfering with the receptors that viruses and bacteria use to infect cells. Newborn infants have no prior exposure to microbes and are particularly vulnerable to infection. Several layers of passive protection are provided by

5141-517: The aqueous portion of the tear film. In nearly all human cultures, crying is associated with tears trickling down the cheeks and accompanied by characteristic sobbing sounds. Emotional triggers are most often sadness and grief , but crying can also be triggered by anger , happiness , fear , laughter or humor , frustration , remorse , or other strong, intense emotions. Emotional tears can also be triggered by listening to music or by reading, watching or listening to various forms of media. Crying

5238-453: The body can manufacture. When B or T cells encounter their related antigens they multiply and many "clones" of the cells are produced that target the same antigen. This is called clonal selection . Both B cells and T cells carry receptor molecules that recognize specific targets. T cells recognize a "non-self" target, such as a pathogen, only after antigens (small fragments of the pathogen) have been processed and presented in combination with

5335-585: The body in pursuit of invading pathogens. Neutrophils are normally found in the bloodstream and are the most abundant type of phagocyte, representing 50% to 60% of total circulating leukocytes. During the acute phase of inflammation , neutrophils migrate toward the site of inflammation in a process called chemotaxis and are usually the first cells to arrive at the scene of infection. Macrophages are versatile cells that reside within tissues and produce an array of chemicals including enzymes, complement proteins , and cytokines. They can also act as scavengers that rid

5432-440: The body of worn-out cells and other debris and as antigen-presenting cells (APCs) that activate the adaptive immune system. Dendritic cells are phagocytes in tissues that are in contact with the external environment; therefore, they are located mainly in the skin, nose, lungs, stomach, and intestines. They are named for their resemblance to neuronal dendrites , as both have many spine-like projections. Dendritic cells serve as

5529-654: The border between innate and adaptive immunity. On one hand, γδ T cells are a component of adaptive immunity as they rearrange TCR genes to produce receptor diversity and can also develop a memory phenotype. On the other hand, the various subsets are also part of the innate immune system, as restricted TCR or NK receptors may be used as pattern recognition receptors . For example, large numbers of human Vγ9/Vδ2 T cells respond within hours to common molecules produced by microbes, and highly restricted Vδ1+ T cells in epithelia respond to stressed epithelial cells. A B cell identifies pathogens when antibodies on its surface bind to

5626-476: The brain controls emotions. Scientists believe PBA results from prefrontal cortex damage. PBA often involves crying. Hence, PBA is mistakable for depression. But PBA is neurological; depression is psychological. Patients with PBA do not experience typical depression symptoms like sleep disturbances or appetite loss. Immune system The immune system is a network of biological systems that protects an organism from diseases . It detects and responds to

5723-776: The brakes on NK cells. Inflammation is one of the first responses of the immune system to infection. The symptoms of inflammation are redness, swelling, heat, and pain, which are caused by increased blood flow into tissue. Inflammation is produced by eicosanoids and cytokines , which are released by injured or infected cells. Eicosanoids include prostaglandins that produce fever and the dilation of blood vessels associated with inflammation and leukotrienes that attract certain white blood cells (leukocytes). Common cytokines include interleukins that are responsible for communication between white blood cells; chemokines that promote chemotaxis ; and interferons that have antiviral effects, such as shutting down protein synthesis in

5820-520: The cells die most migrate from the blood to the tissues, mainly the intestines and lungs, where pathogens are most likely to be encountered. Some monocytes leave the blood circulation and migrate to the muscles where they differentiate and become macrophages . These cells differentiate into two types: proliferative macrophages, which are responsible for increasing the number of stem cells and restorative macrophages, which are involved their maturing to muscle cells. The immune system, particularly

5917-653: The components of the immune system are inactive. The ability of the immune system to respond to pathogens is diminished in both the young and the elderly , with immune responses beginning to decline at around 50 years of age due to immunosenescence . In developed countries , obesity , alcoholism , and drug use are common causes of poor immune function, while malnutrition is the most common cause of immunodeficiency in developing countries . Diets lacking sufficient protein are associated with impaired cell-mediated immunity, complement activity, phagocyte function, IgA antibody concentrations, and cytokine production. Additionally,

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6014-589: The conditions in their environment, such as pH or available iron. As a result, the probability that pathogens will reach sufficient numbers to cause illness is reduced. Microorganisms or toxins that successfully enter an organism encounter the cells and mechanisms of the innate immune system. The innate response is usually triggered when microbes are identified by pattern recognition receptors , which recognize components that are conserved among broad groups of microorganisms, or when damaged, injured or stressed cells send out alarm signals, many of which are recognized by

6111-404: The different roles of the two types of T cell. A third, minor subtype are the γδ T cells that recognize intact antigens that are not bound to MHC receptors. The double-positive T cells are exposed to a wide variety of self-antigens in the thymus , in which iodine is necessary for its thymus development and activity. In contrast, the B cell antigen-specific receptor is an antibody molecule on

6208-431: The formation of a membrane attack complex . The adaptive immune system evolved in early vertebrates and allows for a stronger immune response as well as immunological memory , where each pathogen is "remembered" by a signature antigen. The adaptive immune response is antigen-specific and requires the recognition of specific "non-self" antigens during a process called antigen presentation . Antigen specificity allows for

6305-419: The functions of specialized cells (located in the thymus and bone marrow) is to present young lymphocytes with self antigens produced throughout the body and to eliminate those cells that recognize self-antigens , preventing autoimmunity. Common autoimmune diseases include Hashimoto's thyroiditis , rheumatoid arthritis , diabetes mellitus type 1 , and systemic lupus erythematosus . Hypersensitivity

6402-420: The generation of responses that are tailored to specific pathogens or pathogen-infected cells. The ability to mount these tailored responses is maintained in the body by "memory cells". Should a pathogen infect the body more than once, these specific memory cells are used to quickly eliminate it. The cells of the adaptive immune system are special types of leukocytes, called lymphocytes. B cells and T cells are

6499-527: The host cell. Growth factors and cytotoxic factors may also be released. These cytokines and other chemicals recruit immune cells to the site of infection and promote the healing of any damaged tissue following the removal of pathogens. The pattern-recognition receptors called inflammasomes are multiprotein complexes (consisting of an NLR, the adaptor protein ASC, and the effector molecule pro-caspase-1) that form in response to cytosolic PAMPs and DAMPs, whose function

6596-635: The immune system as well, most notably prolactin , growth hormone and vitamin D . Although cellular studies indicate that vitamin D has receptors and probable functions in the immune system, there is no clinical evidence to prove that vitamin D deficiency increases the risk for immune diseases or vitamin D supplementation lowers immune disease risk. A 2011 United States Institute of Medicine report stated that "outcomes related to ... immune functioning and autoimmune disorders , and infections ... could not be linked reliably with calcium or vitamin D intake and were often conflicting." The immune system

6693-450: The immune system can cause autoimmune diseases , inflammatory diseases and cancer . Immunodeficiency occurs when the immune system is less active than normal, resulting in recurring and life-threatening infections. In humans, immunodeficiency can be the result of a genetic disease such as severe combined immunodeficiency , acquired conditions such as HIV / AIDS , or the use of immunosuppressive medication . Autoimmunity results from

6790-448: The immune system. Conversely, non-self molecules are those recognized as foreign molecules. One class of non-self molecules are called antigens (originally named for being anti body gen erators) and are defined as substances that bind to specific immune receptors and elicit an immune response. Several barriers protect organisms from infection, including mechanical, chemical, and biological barriers. The waxy cuticle of most leaves,

6887-699: The individual's own cells, marking them for destruction. This is also called antibody-dependent (or cytotoxic) hypersensitivity, and is mediated by IgG and IgM antibodies. Immune complexes (aggregations of antigens, complement proteins, and IgG and IgM antibodies) deposited in various tissues trigger Type III hypersensitivity reactions. Type IV hypersensitivity (also known as cell-mediated or delayed type hypersensitivity ) usually takes between two and three days to develop. Type IV reactions are involved in many autoimmune and infectious diseases, but may also involve contact dermatitis . These reactions are mediated by T cells , monocytes , and macrophages . Inflammation

6984-425: The inflammatory response. They are most often associated with allergy and anaphylaxis . Basophils and eosinophils are related to neutrophils. They secrete chemical mediators that are involved in defending against parasites and play a role in allergic reactions, such as asthma . Innate lymphoid cells (ILCs) are a group of innate immune cells that are derived from common lymphoid progenitor and belong to

7081-497: The initiation of Th1 immune responses. During wake periods, differentiated effector cells, such as cytotoxic natural killer cells and cytotoxic T lymphocytes, peak to elicit an effective response against any intruding pathogens. Anti-inflammatory molecules, such as cortisol and catecholamines , also peak during awake active times. Inflammation would cause serious cognitive and physical impairments if it were to occur during wake times, and inflammation may occur during sleep times due to

7178-437: The innate and adaptive immune responses and help determine which immune responses the body makes to a particular pathogen. These cells have no cytotoxic activity and do not kill infected cells or clear pathogens directly. They instead control the immune response by directing other cells to perform these tasks. Helper T cells express T cell receptors that recognize antigen bound to Class II MHC molecules. The MHC:antigen complex

7275-886: The innate component, plays a decisive role in tissue repair after an insult . Key actors include macrophages and neutrophils , but other cellular actors, including γδ T cells , innate lymphoid cells (ILCs), and regulatory T cells (Tregs), are also important. The plasticity of immune cells and the balance between pro-inflammatory and anti-inflammatory signals are crucial aspects of efficient tissue repair. Immune components and pathways are involved in regeneration as well, for example in amphibians such as in axolotl limb regeneration . According to one hypothesis, organisms that can regenerate ( e.g. , axolotls ) could be less immunocompetent than organisms that cannot regenerate. Failures of host defense occur and fall into three broad categories: immunodeficiencies, autoimmunity, and hypersensitivities. Immunodeficiencies occur when one or more of

7372-433: The innate immune system have pattern recognition receptors, which detect infection or cell damage, inside. Three major classes of these "cytosolic" receptors are NOD–like receptors , RIG (retinoic acid-inducible gene)-like receptors , and cytosolic DNA sensors. Some leukocytes (white blood cells) act like independent, single-celled organisms and are the second arm of the innate immune system. The innate leukocytes include

7469-401: The legend. The expression comes from an ancient anecdote that crocodiles weep for the victims they are eating. A collection of proverbs attributed to Plutarch suggests that the phrase "crocodile tears" was well known in antiquity: comparing the crocodile's behaviour to people who desire or cause the death of someone, but then publicly lament for them. The story is given a Christian gloss in

7566-423: The lifetime of an animal, these memory cells remember each specific pathogen encountered and can mount a strong response if the pathogen is detected again. T-cells recognize pathogens by small protein-based infection signals, called antigens, that bind to directly to T-cell surface receptors. B-cells use the protein, immunoglobulin, to recognize pathogens by their antigens. This is "adaptive" because it occurs during

7663-415: The lifetime of an individual as an adaptation to infection with that pathogen and prepares the immune system for future challenges. Immunological memory can be in the form of either passive short-term memory or active long-term memory. The immune system is involved in many aspects of physiological regulation in the body. The immune system interacts intimately with other systems, such as the endocrine and

7760-459: The loss of the thymus at an early age through genetic mutation or surgical removal results in severe immunodeficiency and a high susceptibility to infection. Immunodeficiencies can also be inherited or ' acquired '. Severe combined immunodeficiency is a rare genetic disorder characterized by the disturbed development of functional T cells and B cells caused by numerous genetic mutations. Chronic granulomatous disease , where phagocytes have

7857-454: The major types of lymphocytes and are derived from hematopoietic stem cells in the bone marrow . B cells are involved in the humoral immune response , whereas T cells are involved in cell-mediated immune response . Killer T cells only recognize antigens coupled to Class I MHC molecules, while helper T cells and regulatory T cells only recognize antigens coupled to Class II MHC molecules. These two mechanisms of antigen presentation reflect

7954-509: The microbe, they activate their protease activity, which in turn activates other complement proteases, and so on. This produces a catalytic cascade that amplifies the initial signal by controlled positive feedback . The cascade results in the production of peptides that attract immune cells, increase vascular permeability , and opsonize (coat) the surface of a pathogen, marking it for destruction. This deposition of complement can also kill cells directly by disrupting their plasma membrane via

8051-408: The milieu of hormones produced at this time (leptin, pituitary growth hormone, and prolactin) supports the interactions between APCs and T-cells, a shift of the T h 1/T h 2 cytokine balance towards one that supports T h 1, an increase in overall T h cell proliferation, and naïve T cell migration to lymph nodes. This is also thought to support the formation of long-lasting immune memory through

8148-412: The mother. During pregnancy, a particular type of antibody, called IgG , is transported from mother to baby directly through the placenta , so human babies have high levels of antibodies even at birth, with the same range of antigen specificities as their mother. Breast milk or colostrum also contains antibodies that are transferred to the gut of the infant and protect against bacterial infections until

8245-462: The negative consequences of sleep deprivation, sleep and the intertwined circadian system have been shown to have strong regulatory effects on immunological functions affecting both innate and adaptive immunity. First, during the early slow-wave-sleep stage, a sudden drop in blood levels of cortisol , epinephrine , and norepinephrine causes increased blood levels of the hormones leptin , pituitary growth hormone , and prolactin . These signals induce

8342-506: The newborn can synthesize its own antibodies. This is passive immunity because the fetus does not actually make any memory cells or antibodies—it only borrows them. This passive immunity is usually short-term, lasting from a few days up to several months. In medicine, protective passive immunity can also be transferred artificially from one individual to another. When B cells and T cells are activated and begin to replicate, some of their offspring become long-lived memory cells. Throughout

8439-484: The older story that crocodiles wept during and after eating a man, repeating the standard Christian moral that this signified a kind of fake repentance like Judas weeping after betraying Jesus. Shakespeare regularly refers to the concept. He uses both of Topsell's versions of the motive, as a trick and as fake repentance. A prominent example is in Othello , Act IV, Scene i, in which Othello convinces himself that his wife

8536-452: The organism. If a pathogen breaches these barriers, the innate immune system provides an immediate, but non-specific response. Innate immune systems are found in all animals . If pathogens successfully evade the innate response, vertebrates possess a second layer of protection, the adaptive immune system , which is activated by the innate response. Here, the immune system adapts its response during an infection to improve its recognition of

8633-507: The pathogen. This improved response is then retained after the pathogen has been eliminated, in the form of an immunological memory , and allows the adaptive immune system to mount faster and stronger attacks each time this pathogen is encountered. Both innate and adaptive immunity depend on the ability of the immune system to distinguish between self and non-self molecules . In immunology, self molecules are components of an organism's body that can be distinguished from foreign substances by

8730-434: The presence of melatonin . Inflammation causes a great deal of oxidative stress and the presence of melatonin during sleep times could actively counteract free radical production during this time. Physical exercise has a positive effect on the immune system and depending on the frequency and intensity, the pathogenic effects of diseases caused by bacteria and viruses are moderated. Immediately after intense exercise there

8827-532: The same receptors as those that recognize pathogens. Innate immune defenses are non-specific, meaning these systems respond to pathogens in a generic way. This system does not confer long-lasting immunity against a pathogen. The innate immune system is the dominant system of host defense in most organisms, and the only one in plants. Cells in the innate immune system use pattern recognition receptors to recognize molecular structures that are produced by pathogens. They are proteins expressed, mainly, by cells of

8924-497: The seven animals were seen "weeping", leading to the conclusion that the story describes a real phenomenon. The researchers suggest that the "weeping" may be caused by the hissing of warm air during feeding, which is forced through the sinuses, stimulating the animals' tear glands into emptying fluid into the eye. The phrase gives its name to Bogorad's syndrome , colloquially "crocodile tears syndrome", an uncommon consequence of recovery from Bell's palsy where faulty regeneration of

9021-399: The space between the eyeball and the lids. When the eyes blink, the lacrimal fluid is spread across the surface of the eye. Lacrimal fluid gathers in the lacrimal lake which is found in the medial part of the eye. The lacrimal papilla is an elevation in the inner side of the eyelid, at the edge of the lacrimal lake. The lacrimal canaliculi open into the papilla. The opening of each canaliculus

9118-503: The target cell's plasma membrane , allowing ions , water and toxins to enter. The entry of another toxin called granulysin (a protease) induces the target cell to undergo apoptosis . T cell killing of host cells is particularly important in preventing the replication of viruses. T cell activation is tightly controlled and generally requires a very strong MHC/antigen activation signal, or additional activation signals provided by "helper" T cells (see below). Helper T cells regulate both

9215-422: The tears are not enveloped in a hydrophobic film coat, leading to tears spilling onto the face. Treatment for dry eyes to compensate for the loss of tear film include eye-drops composed of methyl cellulose or carboxy- methyl cellulose or hemi-cellulose in strengths of either 0.5% or 1% depending upon the severity of drying up of the cornea. Familial dysautonomia is a genetic condition that can be associated with

9312-482: The tears help rid of the excess salt that they take in with their food. According to Adam Britton , It is difficult to trace the origin of this particular myth, but it's easy to see why it has become so popular – for an apparently remorseless creature such as a crocodile to actually weep over its victims is a memorable irony which has inspired considerable prose and created a phrase which is still popular today. In 2006, neurologist Malcolm Shaner, assisted by Kent Vliet,

9409-419: Was introduced through Latin . While crocodiles do have tear ducts, they weep to lubricate their eyes, typically when they have been out of water for a long time and their eyes begin to dry out. However, evidence suggests this could also be triggered by feeding. Bogorad's syndrome , a condition which causes sufferers to shed tears while consuming food, has been labelled "crocodile tears syndrome" in reference to

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